The invention relates to a device for directing a radiation beam towards a recording surface of a rotating disc for recording and/or scanning a recording track without mechanical contact. Two elongnate leaf springs are arranged one above the other viewed along the optical axis of the objective and have their ends secured to the frame and to the objective holder to allow movements of the objective along the optical axis. First actuator means secured to the objective holder and second actuator means secured to the frame cooperate electromagnetically with one another via an air gap for the controlled movement of the objective.
Such a device is known from U.S. Pat. No. 4,557,564, hereby incorporated by reference. The known device has a frame constructed as a pivotal arm and is intended for use in an optical-disc player. The pivotal arm forms part of a pivotal-arm device and is capable of performing limited pivotal movements about a pivotal axis which extends parallel to the optical axis of the objective. The objective holder is supported by the leaf springs to allow upward and downward movements along the optical axis of the objective, so as to enable the radiation beam to be maintained constantly in focus on the recording surface of the optical disc. In order to enable the recording track on the disc to be followed by the objective in radial directions relative to the axis of rotation of the optical disc, said pivotal arm is driven and the pivotal movements of the pivotal arm are transmitted to the objective by the leaf springs.
The known device comprises opto-electronic means for automatically following the recording track of the rotating disc during operation. The frame, i.e. the pivotal arm, must then be pivoted bodily to enable the recording track to be followed. This presents no problem for low-frequency tracking movements, but the objective must also be capable of rapidly performing small radial displacements with a high frequency to enable local deviations in the position of the recording track to be followed. In this last-mentioned respect it is to be noted that on account of the size of the arm and the rather unfavourable radial transmission of the leaf springs the servo-bandwidth attainable with the known device is too limited to follow the recording track with a very high accuracy.
A known solution to the problem is use of a slide for coarsely following the recording track and an actuator arranged on the slide for more accurately following the track. However, this solution cannot readily be used in all known automatic tracking systems.